This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 10-278759, filed Sep. 30, 1998; and No. 11-162630, field Jun. 9, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a fuel cell, particularly, to a fuel cell readily available in a reduced size.
Fuel cells are nowadays attracting attentions because of their efficient energy conversion into electricity.
Fuel cells are classified according to the kind of electrolyte and fuel employed. Those which depend on a gaseous fuel are classified as phosphoric acid fuel cells, molten carbonate salt fuel cells, solid electrolyte fuel cells, and alkaline electrolyte fuel cells. Those which depend on liquid fuel are classified as methanol fuel cells and hydrazine fuel cells. These fuel cells are intended to supply electric power to large machines, and they need auxiliary equipment such as compressors and pumps to introduce a fuel gas or liquid and an oxidant gas into them. In addition to their complex structure, they consume electric power for the introduction of fuel and oxidant into them.
The current trend in the fields of office automation (OA) equipment, audio systems, and radio sets is toward miniaturization and portability resulting from the development of semiconductor technologies. This goal is achieved by using handy primary or secondary batteries as the power source. However, batteries are inherently limited in operating time and hence OA equipment depending on them for power supply is necessarily limited in operating time. Primary batteries are rather short in operating time for their weight and hence they do not suit portable equipment. On the other hand, secondary batteries need a power source for recharging which takes a long time. This limit the site where OA equipment can be used and also limits the length of service time. Thus, conventional primary and secondary batteries cannot run small machines for a long period of time, and there is an increasing demand for long-life batteries.
In order to meet this requirement, the present inventors propose that conventional primary and secondary batteries be replaced by fuel cells. Fuel cells offer the advantage of generating electricity continuously as long as they are supplied with fuel and oxidant. Therefore, they contribute to the size reduction and power saving of OA equipment.
Fuel cells employ air as the oxidant and hence they are not restricted in site and time for their use as far as the oxidant is concerned. However, they need a large volume of gaseous fuel with a low density even though the power consumption of OA equipment is small. This is unfavorable to the size reduction of power source. By contrast, liquid fuel has a higher density than gaseous fuel and hence extremely favorable to fuel cells for small-size machines. If fuel cells using liquid fuel are available in small size, they would be able to run small-sized machines for a long time. A hindrance to realization of such a small-sized power system is the necessity for pumps and blowers to feed liquid fuel to the fuel cell proper. The resulting power system is complex in structure and large in size.
Methanol fuel cells (which use methanol as liquid fuel) are explained below. They are broadly classified into liquid feed type and vapor feed type according to the type of fuel feeding. The former is so designed as to feed liquid fuel in the form of liquid, and the latter is so designed as to feed liquid fuel in the form of gas after evaporation. A fuel cell of liquid feed type is supplied with methanol which is circulated under pressure by a pump through the methanol tank and fuel cell proper. Therefore, it necessarily needs a pump for fuel supply. By contrast, in the case of a fuel cell of vapor feed type, methanol (as a liquid fuel) is introduced into a vaporizer by a pump and then methanol vapor is fed to the fuel cell proper by a blower. Unconsumed methanol vapor discharged from the outlet of the fuel electrode is recycled to the methanol tank through a condenser. This process needs a complex system which is not suitable for small-sized machines. By the way, a fuel cell for practical use is usually composed of stacked unit cells. The disadvantage of this construction is that the fuel being fed under pressure by a pump or blower fluctuates in flow through stacked unit cells, causing variation in performance from one cell to another.
One way to address this problem is to utilize capillary action to feed liquid fuel. In a fuel cell of this type, liquid fuel is fed to the fuel electrode from the fuel tank by capillary action. Therefore, it dispenses with a fuel pump, unlike fuel cells of liquid feed type, and hence is suitable for size reduction.
However, a fuel cell of this type still has the disadvantage of being poor in performance due to low electrode reactivity. In addition, it poses another problem of cross-over, which is a phenomenon that an organic liquid fuel (such as methanol) passes through the electrolyte membrane to reach the oxidant electrode, if a proton-conducting solid polymer such as perfluorosulfonic acid (available under a trade name of xe2x80x9cNafionxe2x80x9d from Du Pont) is used as electrolyte.
An object of the present invention is to provide a fuel cell fed with liquid fuel by capillary action, which is characterized by the increased fuel electrode reactivity and the reduced cross-over (a phenomenon that an organic liquid fuel passes through the electrolyte membrane to reach the oxidant electrode).
Another object is to provide a fuel cell that permits simplifying the liquid fuel supply system and also permits miniaturizing the cell size while maintaining a high performance.
According to a first aspect of the present invention, there is provided a fuel cell to produce an electromotive force through the reaction between a liquid fuel and an oxidant gas, the fuel cell comprising a fuel electrode, an electrolyte plate positioned adjacent to the fuel electrode, an oxidant electrode positioned adjacent to the electrolyte plate and opposite to the fuel electrode, and a fuel evaporating portion positioned adjacent to the surface opposite to that surface of the fuel electrode which is in contact with the electrolyte plate.
According to a second aspect of the present invention, there is provided a fuel cell to produce an electromotive force through the reaction between a liquid fuel and an oxidant gas, the fuel cell comprising a first power generating section and a second power generating section, which are placed on top of the other, with a separator interposed therebetween, the first power generating section being composed of a first fuel electrode, a first fuel evaporating layer, a first electrolyte plate, and a first oxidant electrode, which are placed sequentially one over another, the second power generating section being composed of a second fuel electrode, a second fuel evaporating layer, a second electrolyte plate, and a second oxidant electrode, which are placed sequentially one over another, the first and second fuel evaporating layers being fed with the liquid fuel through a liquid fuel passage which is formed adjacently to both of the first and second power generating sections.
According to a third aspect of the present invention, there is provided a fuel cell, comprising:
an oxidant electrode;
an electrolyte plate laminated on the oxidant electrode;
a fuel electrode laminated on the electrolyte plate;
a liquid fuel holding section laminated on the fuel electrode; and
a liquid fuel evaporating portion positioned to be in contact with both the fuel electrode and the liquid fuel holding portion.
According to a fourth aspect of the present invention, there is provided a fuel cell, comprising:
an oxidant electrode;
an electrolyte plate laminated on the oxidant electrode;
a fuel electrode laminated on the electrolyte plate;
a liquid fuel holding portion;
a plurality of unit cells each having a liquid fuel evaporating portion positioned in contact with both the fuel electrode and the liquid fuel holding portion; and
a liquid fuel source connected to each of the plurality of unit cells through the liquid fuel holding portion.
According to a fifth aspect of the present invention, there is provided a fuel cell, comprising:
an electrolyte plate;
an oxidant electrode and a fuel electrode mounted to have the electrolyte plate interposed therebetween;
a liquid fuel evaporating section mounted on the fuel electrode; and
a liquid fuel holding portion positioned away from the fuel electrode by a distance not larger than 1 cm and connected to the liquid fuel evaporating portion.
According to a sixth aspect of the present invention, there is provided a fuel cell, comprising:
an electrolyte plate;
an oxidant electrode and a fuel electrode arranged to have the electrolyte plate interposed therebetween; and
a liquid fuel holding portion for holding the fuel supplied to the fuel electrode, wherein the fuel held by the liquid fuel holding portion is evaporated by the reaction heat of the cell reaction.
Further, according to a seventh aspect of the present invention, there is provided a fuel cell, comprising:
an oxidant electrode;
an electrolyte plate laminated on the oxidant electrode;
a fuel electrode laminated on the electrolyte plate; and
a liquid fuel permeating-evaporating member mounted on the fuel electrode and having a first side on the side of the fuel electrode and a second side facing the first side.
The liquid permeating-evaporating member may include a porous plate having a densifying treatment applied to the surface of the first side. A plurality of through-holes can extend across the first side and the second side may be formed in the porous plate. The surface of the first side can be in contact with the fuel electrode.
In this case, the through-holes may have a predetermined diameter and may be regularly arranged. Further, a porous body may be located in the through-hole, and the capillary phenomenon of the porous body may be smaller than that of the porous plate forming the liquid fuel permeating-evaporating member.
The diameter of the through-hole can be gradually increased in the flowing direction of the liquid fuel.
The number of the through-holes can be gradually increased in the flowing direction of the liquid fuel.
The liquid fuel permeating-evaporating member can be formed of a porous plate having a densifying treatment applied to the surface on a first side thereof. A plurality of recesses may be formed on the surface of the first side, and the surface of the first side may be contact with the fuel electrode.
The liquid fuel permeating-evaporating member may include a first plate mounted on the first side and provided with a plurality of through-holes, and a flat second plate laminated on the first plate with a spacer interposed therebetween. The first plate may be brought into contact with the fuel electrode.
The liquid fuel permeating-evaporating member may be formed of a porous plate. The average pore diameter of the porous plate can be gradually increased from the second side toward the first side, and the surface of the first side can be contact with the fuel electrode.
The liquid fuel permeating-evaporating member may be formed of a porous plate. A wet-proofing treatment can be applied to the first side, a hydrophilic treatment can be applied to the second side, and the surface of the first side can be contact with the fuel electrode.
The liquid fuel permeating-evaporating member may include a porous plate arranged on the second side and a gas-liquid separating membrane arranged in contact with both the porous plate and the fuel electrode.
The liquid fuel permeating-evaporating member may be formed of a dense plate and may include grooves formed on the surface in contact with the gas-liquid separating membrane and extending in the flowing direction of the liquid fuel.
The liquid fuel permeating-evaporating member may include a dense plate arranged on the second side and a frame member arranged in contact with both the dense plate and the fuel electrode. Grooves extending in the flowing direction of the liquid fuel can be formed on that surface of the dense plate which is in contact with the frame member.
In this case, the width of the groove formed in the dense plate can be continuously diminished in the flowing direction of the liquid fuel, and the depth of the groove can be continuously diminished in the flowing direction of the liquid fuel.
The liquid fuel permeating-evaporating member may be formed of a dense plate having a plurality of through-holes. Grooves communicating with the through-holes can be formed in the surface on the second side of the dense plate, and the grooves can communicate with the inlet port of the liquid fuel.
The liquid fuel permeating-evaporating member may consist of a plurality of hollow fibers differing from each other in length and bundled together to form a plate-like configuration. The plurality of hollow fibers can extend in the flowing direction of the liquid fuel.
The liquid fuel permeating-evaporating member may include a fuel evaporating portion arranged on the first side and a fuel permeating portion arranged in contact with both the fuel evaporating portion and the fuel electrode. The fuel permeating portion may have at least two sub-fuel permeating sections serving to connect the fuel inlet port to the fuel evaporating portion.
The liquid fuel permeating-evaporating member may include a hollow plate arranged on the second side and a frame member arranged on the first side in contact with the hollow plate. The hollow plate may have a plurality openings on the surface in contact with the frame member and the frame member can be in contact with the fuel electrode.
In each case, the liquid fuel within the liquid fuel permeating-evaporating member can be evaporated by the reaction heat of the cell reaction.
The fuel cell of the present invention does not need any driving unit, such as pump, to feed fuel, because it introduces liquid fuel into the cell by capillary action. In addition, it does not need any auxiliary equipment, such as fuel evaporator, because the liquid fuel introduced into it is vaporized by the reaction heat in the fuel evaporating layer. The vaporized fuel in the fuel evaporating layer is kept almost saturated, so that as much fuel as consumed is vaporized from the fuel permeating layer and as much fuel as vaporized is introduced into the cell by the capillary action. Since fuel is supplied in proportion to its consumption, there is very little fuel which is discharged unreacted. This obviates the necessity of a subsystem to treat discharged fuel, unlike the conventional liquid fuel cell. Thus, the fuel cell of the present invention can feed liquid fuel smoothly without requiring such auxiliary equipment as a pump, a blower, a fuel evaporator and a condenser, and hence it can be made small in size.
The fuel cell of the present invention may be constructed of a plurality of unit cells (which are placed on top of the other to form a stack). In this case, fuel is vaporized in each unit cell, so that fuel is uniformly distributed among unit cells. This prevents unit cells from fluctuating in performance depending on their position in the stack, unlike the conventional fuel cell of evaporation feed type. The feeding of fuel in gaseous form ensures high activity and high performance at the fuel electrode as in the case of the conventional fuel cell of the evaporation feed type. Another advantage of feeding fuel in gaseous form is that it is possible to prevent methanol cross-over which is a problem that arises when a proton-conducting solid polymer membrane, such as perfluorosulfonic acid (xe2x80x9cNafionxe2x80x9d from Du Pont), is used as the electrolyte and a liquid organic fuel, such as methanol, is used as the fuel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.